The use of soft starters for electrical submersible pumps (ESP's) within the past few years has led to an opportunity to increase production by pumping ESP's off, i.e., pumping well fluid levels down to the pump intake in order to obtain maximum production from a well by lowering its bottomhole pressure. Operating an ESP in this mode means that the ESP is continually cycled on and off as the unit is pumped off, shut down for a short period of time to allow the well to partially fill, and then restarted. This could not have been done prior to the use of soft starters since ESP failure was common on restart.
Besides minimizing restarting failures, ESP pump off operation also requires reliable pump off detection and control to not allow an ESP to operate after it has become gas-locked. Failure to shut down a gas locked ESP will result in premature failure due to overheating. Gas locking occurs when an ESP ingests sufficient gas so as to no longer be able to pump fluid to the surface, the result of either large gas bubbles being present in the well fluid or of the pump intake being uncovered at pump off. In accordance with the present invention an ESP pump off controller has been developed to meet the needs of reliably detecting and shutting down an ESP when gas locked or pumped off since existing ESP motor controllers have been proven to inadequately control under these critical conditions.
Existing ESP motor controllers have been adapted from surface motor control packages where motor operation is more stable and motor control is less critical. For example, it not critical for a motor controller to prevent a surface centrifugal pump from running dry since this will not damage the pump or its motor, but a downhole ESP will fail rapidly if it is run after losing fluid flow to the surface. These motor controllers monitor the running current (or power consumption) of the motor and compare it to a manually adjustable, fixed setpoint. When the current drops below this underload setpoint for a prescribed length of time, the motor is shut down.
Experience has shown that this existing method of motor control is unreliable since pumping ESP's are seen to be prematurely shutting down in underload or not at all. The reason for this unreliability is that the manually entered setpoints are often guessed, or at best, based on varying rules of thumb which may have no correlation to what is going on downhole. As a result, setpoints are frequently set too high causing premature shutdown and loss of production or set too low failing to shut the ESP down, causing failure of the ESP and loss of production.
Applicants are not aware of any prior art which, in their judgment as persons being skilled in this particular art, would anticipate or render obvious this novel technique of the present invention; however, for the purposes of fully developing the background of the invention, and establishing the state of the requisite art, the following are set forth: U.S. Pat. Nos. 4,302,157; 4,302,158; 2,774,929; 4,057,365; Petroleum Engineer International, December 1986, pp. 41-44.